A math professor leaves a note for his wife

You must realize that you are 54 years old and I have certain needs which you are no longer able to satisfy. I am otherwise happy with you as a wife and I sincerely hope you will not be hurt or offended to learn that I have rented a hotel room with one of my 18 year old students. I will be home before midnight.

When he gets home around 11:45, he finds a note waiting for him that reads as follows:

Dear Husband,

You, too, are 54 years old and by the time you read this, I will be in a hotel room with our 18 year old pool boy. Being the brilliant mathematician you are, you can easily appreciate the fact that 18 goes into 54 many more times than 54 goes into 18.